A Multi-Input Multi-Output (MIMO) technology plays a very important role in improving a peak rate and a system spectrum utilization rate, so such wireless access technology standard as Long Term Evolution (LTE) and LTE-Advanced (LTE-A) are constructed on the basis of an MIMO+Orthogonal Frequency Division Multiplexing (OFDM) technology.
A performance gain of the MIMO technology is derived from a spatial freedom degree available for a multi-antenna system, so during the standardization, one of the most important developments of the MIMO technology lies in the extension of dimensions.
For a base station antenna system with a conventional Passive Antenna System (PAS) structure, a plurality of antenna ports is arranged horizontally, and a plurality of elements corresponding to each antenna port in a vertical dimension is connected through a radio frequency (RF) cable. Each antenna port corresponds to a separate RF-intermediate frequency-baseband channel. Hence, it is merely able for the conventional MIMO technology to optimize spatial characteristics of each terminal signal in a horizontal dimension by adjusting a relative amplitude or phase of different antenna ports. In the vertical dimension, uniform sector-level beamforming may merely be adopted. Upon the introduction of an Active Antenna System (AAS) technology into a mobile communication system, the base station antenna system has acquired a greater freedom degree in the vertical dimension, so it is able to optimize the signal at a User Equipment (UE) level in a three-dimensional (3D) space.
On the basis of the above-mentioned research, standardization and the development of the antenna technology, the MIMO technology is being developed toward a 3D and massive direction in the industry. A massive MIMO technology will be able to remarkably increase the utilization of a system frequency band, and thereby support a larger number of UEs.
The massive MIMO technology needs a large-scale antenna array. Although it is able for a full-digital array to achieve a maximum spatial resolution as well as optimal Multi-User MIMO (MU-MIMO) performance, this structure needs a large number of analog-to-digital (AD) and digital-to-analog (DA) switching procedures and a large number of complete RF-baseband processing channels, resulting in a huge burden on both the cost and the baseband processing complexity, especially for a high frequency band and a wide bandwidth.